1. Field of the Invention
The present invention generally relates to driving of an electro-absorption optical modulator, and more specifically to a method of and a device for driving an electro-absorption optical modulator and to an optical communications system to which the device is applied.
2. Description of the Related Art
With a sudden increase in the amount of information, there has recently been a demand for a massive increase in storage of an optical communications system. Therefore, a transmission rate, i.e., a modulation rate has been increased. In direct intensity modulation of a laser diode, relatively large chirping (dynamic variations in wavelength due to relaxation vibrations or the like) will limit the transmission rate and the modulation rate. When signal light having chirping is transmitted through an optical fiber having wavelength dispersion (chromatic dispersion), distortion normally occurs in waveform. To avoid this problem, expectations for the use of an external optical modulator resistant to the generation of the chirping are generally rising.
A Mach-Zehnder type optical modulator (MZ modulator) has been developed as a practical external optical modulator. Carrier light having predetermined intensity, which is emitted from a light source, is supplied to the MZ modulator from which signal light subjected to intensity modulation is obtained by a switching operation using light interference. For example, an MZ modulator (described in the technical report LQE89-35 by Kiyono, et al. in Electronic Information Communications Association) using an LiNbO.sub.3 crystal and an MZ modulator (described in the lecture preliminary report, pp. 25-29, by Inoue to the 8th optical symposium) using a compound semiconductor crystal have been reported.
The MZ modulator has frequently pointed-out drawbacks that a relatively large drive voltage is required, automatic bias control for holding an operating point constant is required and a device tends to increase in scale. With the foregoing drawbacks in view, an electro-absorption optical modulator (EA modulator) has been proposed as an external optical modulator which is capable of being driven under low power and is suitable for a size reduction. The EA modulator absorbs carrier light according to an applied voltage to thereby produce signal light subjected to intensity modulation. For example, an EA modulator (described in the technical report LQE95-17 by Yamada, et al. in Electronic Information Communications Association) using a compound semiconductor crystal has been reported.
A practical EA modulator is provided as a semiconductor chip by semiconductor lamination technology. The EA modulator is easily formed integrally with a laser diode used as a carrier light source. Thus, the EA modulator can provide a high output owing to a reduction in loss of coupling between the light source and the modulator and can be reduced in size owing to the integral formation. For example, a semiconductor chip (described in the technical report LQE95-17, pp. 1-6 by Morito, et al. in Electronic Information Communications Association) obtained by monolithically uniting a DFB-LD (Distributed Feed Back Laser Diode) and an EA modulator has been reported.
It became evident that wavelength chirp would be produced due to induced phase-modulated components depending on the type of optical modulator (F. Koyama, and K. Iga: J. Lightwave Technol., LT-6, (1988) pp. 87-93). The amount of the produced wavelength chirp is represented by chirping parameters of the optical modulator on a unity basis. A lightwave or optical pulse after having been transmitted is expanded or compressed according to the positive or negative chirping parameter. When signal light lying within a 1.55 .mu.m band is transmitted through a silica fiber having a zero dispersion wavelength within a 1.3 .mu.m band, the positive chirping parameter provides pulse expansion and the negative chirping parameter provides pulse compression.
It has been reported that the application of suitable chirping to a waveform to be transmitted (prechirping) has dramatically increased a transmission distance (K. Morito, R. Sahara, K. Sato, Y. Kotani, and H. Soda "MQW Modulator Integrated DFB Lasers for Multigigabit Transmission Systems" Proc. 21st Eur. Conf. on Opt. Comm. (ECOC 1995--Brussels) Th. B. 2.1, pp. 887-891). Thus, the performance of a system to which the optical modulator is applied, is improved on a great leap basis by making it possible to arbitrarily set the chirping parameter of the optical modulator.